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This reaction supposedly passes through an epoxide intermediate. Can someone please explain why this is necessary?


  • $\begingroup$ Well, these types of questions are always answered with: run a mechanistic study and check. If it's the way it is, then it's necessary because that's what nature has chosen. To be fair, the mechanism isn't clearly defined here either because the reaction conditions are not clearly defined. $\endgroup$ – Zhe May 22 '18 at 15:36
  • $\begingroup$ @Zhe This sites tries to show a possible expoxide mechanism-www2.chemistry.msu.edu/faculty/reusch/virttxtjml/rearrang.htm Would you like to say anything about it? $\endgroup$ – tatan May 22 '18 at 15:37
  • $\begingroup$ @Zhe I read the mechanism but couldn't understand it fully... $\endgroup$ – tatan May 22 '18 at 15:41
  • $\begingroup$ I don't have anything to say about it. It's conceivable, and we don't tell nature what to do because I think it's better. It does what it does, and we try to reconcile what happens with our simplistic rules. $\endgroup$ – Zhe May 22 '18 at 15:43
  • 2
    $\begingroup$ Yes, there are several simple mechanistic possibilities, and all of them may operate to some extent depending on conditions. Only way to test would be to run experiments. Though, presumably, if you assert that the mechanism is such and such, you have run the experiments to rule out the others. $\endgroup$ – Zhe May 22 '18 at 17:14

As I mentioned above, during an isotope labelled mechanistic study, the formation of epoxide intermediate is suggested to explain the scramble isotopic carbon (between carbonyl carbon and neighboring carbon). The detailed arrow pushing mechanism is given bellow:


These steps are subjected to change according to many factors including the reaction conditions and nature of the diol used as correctly pointed out by Zhe and aforementioned MSU website in your comments, which states:

Many factors must be considered when analyzing the course of a pinacol rearrangement. These include:

• Which hydroxyl group is lost as water? or Which intermediate carbocation is more stable?

• What is the inherent shifting tendency (migratory aptitude) of different substituent groups? • What is the influence of steric hindrance and other strain factors on the rearrangement? • Are epoxides formed as intermediates in the pinacol rearrangement?

• Does product stability govern the outcome of competing rearrangements?

• Do the reaction conditions (i.e. type of acid, concentration, solvent and temperature) influence the course of rearrangement?

A reference for the rearrangement through an epoxy intermediate:

C. A. Bunton, T. Hadwick, D. R. Llewellyn, Y. Pocker, Tracer studies in alcohols. Part III. Intermediates in the pinacol–pinacone rearrangement, J. Chem. Soc., 1958, 403-408 [DOI: 10.1039/JR9580000403] (http://pubs.rsc.org/en/Content/ArticleLanding/1958/JR/JR9580000403#!divAbstract).


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